1. Field of the Invention
The invention described in this specification is primarily intended to be utilized in conjunction with the perforating of coatings by means of a laser beam and is particularly directed to a process for producing a large number of small holes in coatings applied to the surface of vacuum transfer drums used in electrophotographic reproduction apparatus.
2. Description of the Prior Art
It is known to utilize a laser for the purpose of perforating metallic materials. One such procedure is disclosed in U.S. Pat. No. 4,857,698 where a pulsed Nd:YAG laser is used to perforate a flat metal sheet. A removable tape is placed on the side of the sheet opposite to that of laser impact to reduce the formation of burrs adjacent the perforations.
U.S. Pat. No. 4,156,807 discloses an electron beam drilling process in which the workpiece surface first to be penetrated by the beam is coated with a stop-off material of high temperature resistance so that the hot molten spatter will be deposited on the stop-off layer as opposed to the workpiece and thereby prevent the spatter from becoming welded to the workpiece. This stop-off layer, like the tape of U.S. Pat. No. 4,857,698, is removed from the workpiece once all the perforations are formed.
While the above procedures prevent burrs adjacent to the perforations and spatter from being deposited on the workpiece, these procedures do not address the problems of forming multiple holes in nonmetallic substances, such as polyurethane, coated over a metallic substrate material, wicking back of material into a formed perforation, the electrical relationship between a coating and its substrate nor placement of the tape or stop-off material that is impossible or extremely difficult to remove once the perforation process is complete.
While U.S. Pat. No. 5,119,550 addresses the problem of forming multiple holes in non-metallic substances coated over a metallic substrate material and has a tape placement between the coating and substrate to prevent the coating, when initially applied to the substrate from clogging the holes in the substrate, it does not address the problems of polymeric melt, formed during the perforation process, wicking back into the formed perforation.
It is therefore an object of the present invention to provide an apparatus and method for forming very small holes, of uniform size and shape, in the coating of a substrate material without disturbing the electrical relationship between the coating and the substrate.
Another object is to provide an efficient and rapid process for producing a large number of small uniform holes in a polymeric coating covering a metallic drum, by means of a laser beam, to provide a vacuum drum that may be efficiently employed as a vacuum transfer drum in an image forming device.